Systems and techniques for communicating data from a central station to a plurality of subscribers or users are well known in the art. One technique, known as Teletext, involves the transmission of data during the vertical blanking interval of each television field; see, for example, U.S. Pat. Nos. 4,233,628 to Cicora, 3,493,674 to Houghton, 4,052,719 to Hutt et al and 4,205,343 to Barret. A special issue of the IEEE deals specifically with text display systems such as Teletext and Viewdata. This publication appeared in the IEEE Transactions On Consumer Electronics, dated July 1979, Volume CE-25, Number 3. The Teletext system is particularly adapted to the transmission of the same data to many subscribers who may individually select, store and subsequently display information of interest. The Teletext system may be expanded to fill all available television scan lines with data so that a large amount of data can be transmitted to users. This system, however, does not provide a two-way communication link between the central station and subscribers and is not intended for communicating information at different data rates to different individual subscribers in a large group on a private line or circuit demand basis.
In U.S. Pat. No. 4,199,656 to Sayler, a video signal processor is described wherein data is placed on television scan lines and periodically updated as required. A pseudo video scan line is transmitted to receivers and contains a row of information equivalent to about 11 to 13 actual television scan lines and represents an entire packet of information necessary for video display of the row. Reference is made to a technique for capturing and storing rows of video information for repetetive and continuous display. Other known devices for transmitting data on television scan lines are described in U.S. Pat. Nos. 4,115,662 to Y. M. Guinet et al; 3,743,767 to D. Bitzer et al which distributes data bits designated for individual terminals among the several television scan lines. Although useful, these various data transmission techniques are not as flexible and convenient to apply in a television network wherein data at many different rates needs to be delivered to different subscribers together with return of data from the subscribers to the central station on a real time or other prompt basis.
Techniques have been proposed for enabling two-way communication between a central station and a plurality of subscribers. For example, in U.S. Pat. No. 3,751,595 a time division multiplexed signal having a series of frames and a predetermined number of time slots in each frame is transmitted over a single communications circuit connected to subscriber terminals. Information related to a subscriber is tapped off at the subscriber's terminal such as by counting the number of time slots in each frame until the time slots related to the subscriber is received. The subscriber terminal retransmits the time slots not intended for that terminal and tacks on new information for return transmission. U.S. Pat. No. 4,161,786 describes a return transmission system for a plurality of subscribers connected to a common channel.